A piston is an element that separates two chambers (a first chamber and a second chamber), each containing a fluid. For example, in an actuator, the actuator piston is suitable for moving in an actuator cylinder under drive from the fluid contained in the first chamber so as to exert an action on the fluid contained in the second actuator chamber, the fluid in turn exerting action on a piece of equipment that it is desired to actuate.
In order to be effective, the piston must form a leaktight interface between the two chambers, even when the piston is moving in its cylinder.
Nevertheless, in certain situations, it is desirable to maintain fluid flow between the first and second chambers. For example, in aviation turbines, fuel is used as fluid in actuators. The fuel therefore needs to be cooled. This cooling is provided by causing the fuel to flow continuously through the actuators and the pipework. It is therefore necessary to organize a leakage flow rate between the first chamber and the second chamber in order to allow the fuel to flow between those chambers.
By way of example, leakage flow is implemented by piercing a straight hole through the piston, which hole connects together the first and second chambers. Thus, a fluid flow is established between the chambers. Nevertheless, the hole must be of a diameter that is small enough to avoid harmfully decreasing the effectiveness of the piston in operation, while also allowing the fluid to be cooled sufficiently. In practice, this diameter needs to be less than 0.6 millimeters (mm). As a result, in operation, the hole rapidly becomes clogged by polluting particles (contained in the fluid), and this is undesirable.
Alternatively, this leakage flow may be implemented by placing a Lee-jet restrictor within the piston. The restrictor communicates via ducts both with the first chamber and with the second chamber and it presents strainers (filters) and pierced diaphragms through which the fluid passes on passing between the two chambers, thereby serving to reduce the fluid flow rate. Nevertheless, a Lee-jet restrictor is expensive, and it requires accurate positioning within the piston. In addition, it also tends to become clogged after prolonged use, since the strainers are of small size.